Add a keep to generate rubocop todo files

What does this MR do and why?

Due to code changes, some rubocop offenses get automatically fixed over time. Therefore we should regenerate the rubocop todo files periodically to avoid introducing rubocop offenses again in those files.

This will also help us to quickly fix the silenced rubocop offenses messages we get in the f_rubocop channel on Slack.

We're using the housekeeper gem here to generate the .rubocop_todo files, create an MR, and request a review from a random backend reviewer from the review roulette.

For now, we're going to run it locally by following these steps. In the next iteration we'll run this in CI monthly or fortnightly.

See the MR I have created using this Regenerate rubocop todo files (!157740 - merged).

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

Run bundle exec gitlab-housekeeper -d -k Keeps::GenerateRubocopTodos to test out the changes locally.

Related to #468897